A secondary battery is a battery which is able to provide electricity by converting a loss in chemical energy into electrical energy; moreover, it is a battery which is able to store (during charge) chemical energy by converting electrical energy into chemical energy by passing an electrical current in a direction that is opposite to the discharge direction. Among secondary batteries, lithium secondary batteries have higher energy density, so that they are widely used as a power source for notebook personal computers, cellular phones, etc.
In a lithium secondary battery using graphite (C6) as the negative electrode active material, the reaction described by the following formula (1) proceeds at the negative electrode upon discharge:C6Li→C6+Li++e−  (1)
An electron produced by the formula (1) passes through an external circuit, works by an external load, and then reaches the positive electrode. At the same time, a lithium ion (Li+) produced by the formula (1) is transferred through the electrolyte sandwiched between the negative and positive electrodes from the negative electrode side to the positive electrode side by electro-osmosis.
When lithium cobaltate (Li0.4CoO2) is used as a positive electrode active material, a reaction described by the following formula (2) proceeds at the positive electrode upon discharge:Li0.4CoO2+0.6Li++0.6e−→CoO2  (2)
Upon charging the battery, reactions which are reverse to the reactions described by the above formulae (1) and (2) proceed at the negative and positive electrodes. The graphite material in which lithium was intercalated (C6Li) becomes reusable at the negative electrode, while lithium cobaltate (Li0.4CoO2) is regenerated at the positive electrode. Because of this, discharge becomes possible again.
Among lithium secondary batteries, a lithium battery all-solidified by using a solid electrolyte as the electrolyte, uses no combustible organic solvent in the battery; therefore, it is considered to be safe, able to simplify the device and excellent in production cost and productivity. A sulfide-based solid electrolyte is known as a solid electrolyte material used for such a lithium secondary battery.
However, a sulfide-based solid electrolyte material is likely to react with moisture. Because of this, a battery comprising a sulfide-based solid electrolyte material has a problem that a deterioration is likely to be caused to the battery by the generation of hydrogen sulfide, thereby shortening the lifetime of the battery.
Techniques for solving such a unique problem of the sulfide-based solid electrolyte material have been developed so far. Patent Literature 1 discloses a technique for an all-solid lithium secondary battery which uses a sulfide-based solid electrolyte material and has a power generation element in which an oxide layer substantially containing no moisture, which is obtained by oxidation of the sulfide-based solid electrolyte material, is formed in a part where electrolyte-containing layers containing at least the sulfide-based solid electrolyte material is in contact with external air.